The invention relates to a method for applying a coating to a fiber by chemical vapor deposition (CVD).
Fiber reinforced ceramic matrix composites (CMCs) are formed of continuous uniaxial or woven fibers of ceramic material embedded in a ceramic matrix. These materials are designed to have a weak fiber-matrix bond strength so as to increase overall composite strength and toughness. When the CMC is loaded above a stress that initiates cracks in the matrix, the fibers debond from the matrix allowing fiber/matrix sliding without fiber fracture. The fibers can then bridge a matrix crack and transfer load to the surrounding matrix by transferring tensile stresses to frictional interfacial shear forces.
High temperature fiber reinforced CMCs have great potential for use in aircraft and gas turbine engines due to their high strength to weight ratio. The fibers may be in the form of individual filaments, as unidirectionally aligned fiber bundles or tows, or as woven, knitted or braided fabrics. A coating can be applied to the fibers for several purposes such as to protect them during composite processing, to modify fiber-matrix interface stresses and to promote or prevent mechanical and/or chemical bonding of the fiber and matrix. Chemical vapor deposition (CV3) is a common method for applying the fiber coatings.
The coatings must uniformly coat all fibers. While this may be possible when the fibers are individual filaments, it is very difficult to achieve when the fibers are collected into tows and woven, braided or knitted into fabrics. Adjacent and neighboring fibers block the flow of gaseous coating reactants to the fiber surfaces in the tow or cloth and block counterflow of reaction by-product gases away from the fibers. This is particularly true where fiber tows cross over or loop around each other as when mounted on a mandrel for CVD of a coating.
Chemical vapor deposition of a fiber coating is a time consuming and costly operation. The volume capacity of a CVD reactor is limited, and so to increase reactor through-put and decrease coating costs, as much fiber is packed into the reactor as possible for each coating deposition run. The packing of the fiber conflicts with the requirement of loosely arranging the fiber to allow for a uniform coating deposition.
Another problem related to uniformity of coating has to do with fiber-to-fiber and fiber-to-support contact. Any time a fiber is in direct physical contact with another body during coating deposition, the contact will interfere with the coating. Furthermore, contacting fibers are often xe2x80x9cbondedxe2x80x9d together by the coating deposited near a contact region. The coating in a bonded region is easily and commonly damaged when the fiber tow or cloth is removed from the support structure upon completion of the CVD. Consequently, as little physical fiber to fiber contact as possible is desired during coating deposition to minimize the amount of coating bonding and subsequent coating damage.
In one technique, many layers of fiber tow are overlapped onto a support structure such as a fixture, frame or mandrel to provide a high loading of fiber for the deposition furnace. Only part of the desired coating is deposited in a first run. The fiber tow is then removed from the deposition furnace and unwrapped from the support structure. The tow is then rewrapped onto the support structure, which is again loaded into the deposition furnace for a second run. This cycle is repeated several times for a thick coating. The unwrapping and rewrapping of the tow from the support structure is done to randomize contact points and tightly packed areas. The rewrapping method minimizes contact caused defects and non-uniformity caused by tight fiber packing. However, several coating runs are needed to adequately apply the fiber coating, thus reducing coating productivity and increasing coating costs.
Thus, there is a need to provide a method to coat the fibers of a wrapped tow in an efficient and effective manner.
The invention relates to a method for applying a coating to reinforcing fibers arranged into a tow. The tow is aligned with an adjacent separation layer and wound onto a support structure in an interleaved manner so that the separation layer separates a wrap of the tow from an adjacent wrap of the tow. A coating can then be uniformly applied to the reinforcing fibers without defects caused by fiber tow to fiber tow contact. The separation layer can comprise a carbon fiber veil.
In another aspect, the invention relates to wrapping a support structure with alternating layers of separation layer and tow of reinforcing fibers in an interleaved manner so that each wrap of tow is separated from each other wrap of tow. A coating is then applied onto the fibers of the tow in a chemical vapor deposition furnace.
Finally, the invention relates to a composition of matter, comprising a tow of reinforcing fibers and a separation layer with longitudinal axis in parallel alignment with a longitudinal axis of the tow. The tow and the separation layer are wound or wrapped together onto a support structure in an interleaved manner with the separation layer separating a wrap of the tow from an adjacent wrap of the tow.